1. Field of the Invention
The present invention relates to a self-expanding stent for a medical device to be introduced into a cavity of a human or animal body. More particularly, the present invention relates to a braided or woven filament medical stent having a tubular body which is radially expandable and retractable between a first diameter, corresponding to a compressed state of the stent, and a second diameter, greater than the first, corresponding to a resting state of the stent, and is axially expansible and retractable between a first length, corresponding to the working state, and a second length, less than the first, corresponding to the resting state. For purposes herein, the term "self-expanding stent" is intended to mean that, when brought into its working position (i.e., a radially compressed and axially extended position), the stent, once released, tends spontaneously to substantially recover its resting position (i.e., a radially expanded and axially contracted position). Also, the term "medical device to be introduced into a cavity of a human or animal body" as used herein should be taken to mean, for example, luminal endoprostheses, catheters, dilators, grafts and the like which can be introduced, for example, into the vascular, esophageal, urinary, urethral, or biliary cavities and other tubular ducts of the body. Further, the terms "woven", "braided" and "plaited" are used interchangeably herein and are intended to be understood in their broadest sense to require an over-under interlacing of a plurality of filaments.
2. State of the Art
Braided or woven filament medical stents which are used as vascular, esophageal or other dilators have been known for a long time (See, e.g., GB-1,205,743). These stents, formed by a tubular plaited structure of individual filaments include, in the resting state, filaments forming a fairly small angle with respect to a longitudinal axis of the tube (e.g., 30.degree.). Stents with a relatively small directed angle do not offer a sufficient capacity for radial expansion when they are released after having been radially compressed. Their crushing resistance is also insufficient when they are used in cavities of the body whose walls exert a strong radial pressure on the stent introduced.
It has consequently been sought to overcome these drawbacks by providing a plaited structure of individual filaments with a larger angle so that, in the resting state of the stent, the filaments exhibit an angle greater than 45.degree., preferably of at least 60.degree., with respect to a longitudinal axis of the stent (See, e.g., U.S. Pat. Nos. 4,655,711 and 4,954,126). However, stents with a large braiding angle have a great drawback. Indeed, the tubular stent must be extended to two to three times its initial length in order to be capable of being inserted in a vascular introducer. It then occupies from 40 to 50% of the length of the introducer, and stiffens it and renders it difficult to pass through the femoral artery through which introduction into the body is generally initiated. On release, the tubular structure retracts by two to three times in length. It is therefore very difficult to estimate the length which will be deployed in the internal cavity of the body treated. This may lead to serious problems, as it is difficult to estimate the exact location where the stent will be anchored. For example, a length of stent may be deployed beyond a vascular bifurcation and thus lead to undesirable artery closure when the stent is provided with a covering, as is the case for luminal endoprostheses. In another case, if the deployed length is too short, the aneurysm treated will not be closed. Finally, it has been observed that stents with a large plaiting angle do not exhibit good resistance to blood pressure inside the aneurysms treated.
Other filament-based tubular plaited structures, which are used in devices to be introduced into the human body, are also described, in particular, in Patent #EP-A-0,183,372, U.S. Pat. Nos.3,509,883, 5,061,275 and 5,171,262.